The major antigens from four serotypes of Legionella pneumophila are polysaccharides which have been isolated in high yield. No immunogenic cross-reactivity is seen for either of these antigens. Preliminary evidence has shown that these polysaccharides are galactans, the structural studies of which will be pursued by methylation and periodate oxidation. Partial enzymic hydrolysis will attempt to establish further if there is any basic common core for the polysaccharide antigens that may serve as a general protective agent. These studies will be conducted in guinea pigs.